A fieldbus is a specific type of local area network (LAN) that is used to monitor or control one or more pieces of production equipment. The term "Fieldbus" generally refers to an all-digital, two-way communication system that connects control systems to instrumentation. A fieldbus network comprises a plurality of digital devices and control/monitoring equipment that are integrated to provide I/O and control for automated processes. A fieldbus network is typically used in industrial and/or process control applications, such as a factory or manufacturing plant. The physical devices in a fieldbus system are connected by the fieldbus. One example of a fieldbus network is the Fieldbus Foundation fieldbus network.
Foundation Fieldbus, which is the serial communications network created by the Fieldbus foundation, specifically targets the need for robust distributed control in process control environments. Devices connected by a Foundation Fieldbus exchange data between themselves, and thus control a fieldbus process. Devices are given the opportunity to communicate data by a token passing scheme controlled by a device referred to as the "Link Active Scheduler" or LAS (a device used to schedule communication traffic).
The components of a Foundation Fieldbus Network include the following:
1) Link PA1 2) Devices PA1 4) Linkages PA1 5) Loops PA1 6) Schedule PA1 1) sending a new message overwrites the old message in the buffer, and PA1 2) reading from the buffer does not destroy its contents. PA1 1. receipt of the device tag and the network address (data link layer) for the device from an external system manager application., PA1 2. maintenance of the application clock time (system time) distributed by an external system manager application, PA1 3. providing services to device applications for locating and identifying remote devices, and for locating remote application objects. PA1 4. maintenance of system management information for the device in the system management information base (SMIB), PA1 5. executing function blocks according to the schedule contained in the SMIB.
Fieldbus networks may contain one of four types of devices, these being temporary devices, field devices, interface devices, and monitor devices. Temporary devices are devices attached to one of four network addresses reserved for temporary or visitor use. Temporary devices are typically used for configuration and troubleshooting. Field devices are devices that typically comprise function block application processes or, in other words, devices that perform the I/O and control that automates the plant or factory. All field devices are given a permanent address by the system manager when they are attached to the network. Interface devices perform data display and other interface functions for field devices. Like field devices, interface devices are assigned a permanent address, but interface devices do not necessarily contain function block application processes. Finally, monitor devices are devices that are able to listen to network traffic but are not permitted to transmit onto the network. Monitor devices receive no address when attached to the network, and the other network devices are unaware of the monitor's presence.
A field device generally comprises one or more different types of blocks, including function blocks, resource blocks, and transducer blocks. A function block comprises an algorithm and one or more parameters associated with the algorithm. Function blocks model field device functions, such as analog input (AI) functions and PAID (Proportional Integral Derivative) control loop functions, among others. The function block model provides a common structure for defining function block inputs, outputs, algorithms and control parameters. This structure simplifies the identification and standardization of characteristics that are common to function blocks. A collection of one or more function blocks is referred to as a virtual field device (VFD).
A resource block makes visible hardware specific characteristics associated with function block application processes. Resource blocks include a set of contained parameters to represent these characteristics. Resource blocks also include an algorithm that is used to monitor and control the general operation of the physical device hardware.
A transducer block operates to control access to I/O devices through a device independent interface defined for use by function blocks. Transducer blocks insulate function blocks from the specifics of I/O devices, such as sensors, actuators, and switches. Transducer blocks also perform functions, such as calibration and linearization, on I/O data to convert it to a device independent representation
The function block model includes associated standardized definitions used to support function blocks. These definitions include the Object Dictionary (OD) and the Device Description Language (DDL). These definitions support application processes in the definitions and description of their network visible objects, such as function blocks and their parameters.
Each physical device performs a portion of the total system operation by implementing one or more application processes. Application processes perform one or more time-critical functions, such as providing sensor readings or implementing one or more control algorithms. As noted above, these field device functions are modeled by function blocks. The operation of the various function blocks is coordinated through configuration of their external communications, execution schedules, and operating parameters.
Physical devices in a fieldbus system are interconnected by a fieldbus network. Fieldbus networks may be comprised of one or more link segments interconnected by bridges. Communication between physical devices is provided by physical layer and data link layer protocols. Intervening bridges forward messages transparently between links.
A fieldbus network utilizes a four layered communication stack or four layer protocol, including a physical layer, a data link layer, and application layer that is divided into an access sublayer and a messaging sublayer. Transfer of information between field devices is provided by the physical layer and data link layer protocols. The physical layer specifies how signals are sent, the fieldbus data link layer (FDL) specifies how the network access is shared and scheduled among devices, and the application layer defines the message formats available to applications for the exchange of commands. Communications between application processes in the field devices occurs through the application layer protocols. The application layer is divided into two sublayers, an upper sublayer, defamed by the Fieldbus Messaging Specification (FMS), that provides services to application processes, and a lower sublayer, known as the Fieldbus Access Sublayer (FAS), that maps FMS services onto the underlying capabilities of the data link layer.
In fieldbus networks, information is transmitted along the fieldbus in discrete segments commonly referred to as packets. Packets are passed between the field devices connected to the network. Each packet may contain FDL information, FAS information, and FMS information corresponding to the various architectural layers used in the fieldbus implementation.
System management is used in fieldbus networks to coordinate the operation of the various devices in a distributed fieldbus system. System management functions include node address assignment for devices, application clock synchronization, distributed application scheduling across the segment, and support for locating application tags. System management provides the facilities to bring new devices on the segment to an operational state and to control the overall system operation. System management uses FMS to remotely access management information within a device and to directly access the data link layer to carry out its other functions. A single system management entity exists in each device for each data link segment connected to the device. The entity comprises user applications and a system management kernel. The kernel provides a network coordinated and synchronized set of functions.
In order to program a fieldbus network for a certain application, a fieldbus configuration is created and downloaded to the network. A fieldbus application or configuration specifies communication between two or more blocks, such as function blocks. A fieldbus application or configuration also includes accesses, i.e., reads or writes, to various parameters in function blocks. Thus, a fieldbus application or configuration is generally required to access, i.e., read or write, parameters or objects in various blocks.